Among non-destructive and non-contact detection technologies, a technology of acquiring a projection image is simplest. As an example of the technology, there is an X-ray technology, etc. As known well, in case of irradiating a transmittable electromagnetic wave, the X-ray to the sample and then detecting the X-ray transmitting the sample, the detected amount of X-ray varies depending on the absorbed amount of X-ray while the X-ray transmits the 3D sample. On this basis, it is possible to obtain a 2D projection image for the 3D sample.
A terahertz (THz) beam is also an electromagnetic wave having excellent transmission and may transmit various non-conductive materials such as fiber and plastic. In particular, compared to the X-ray, the terahertz beam has relatively lower photon energy and thus does not harm a biological tissue or DNA, thereby obtaining higher biological safety. A non-destructive inspection technology using a THz beam may be used in a security check to check stuffs in a suitcase or whether passengers possess weapons, etc.
Meanwhile, the projection image acquired by using the X-ray or the THz beam as described above may provide only the 2D information on the 3D object. As a technology of more accurately exposing a 3D structure for a sample, a computed tomography (CT) technology, an optical coherence tomography (OCT) technology, or the like have been used. Generally, these technologies have been mostly used to image an in vivo 3D structure in a medical field. In the CT technology, tomographic or 3D images may be reconstructed by obtaining X-ray projection images for a sample at multiple angles and then recombines the images. Further, the OCT technology uses an optical coherence phenomenon to image micro structures in a sample. In particular, the OCT technology may image micro structures in a biological tissue while minimizing a damage to the biological tissue, and as a result has been in the limelight of a medical field.
By the way, as generally well known, the CT technology needs to obtain hundreds to thousands of 2D projection images for a sample at different angles, and therefore it takes much time to obtain a CT image. That is, a 3D shape detection technology using the CT method requires a too long image acquisition time to be applied to industries other than the medical field. Meanwhile, in the OCT technology, researches for improving a light source output, stability, a speed, etc., have been actively conducted. However, since a 3D imaging depth is only several mm, the OCT technology may be restrictively applied to a retinal diagnosis, an endoscope technology, or the like.
Today, a necessity for a 3D imaging technology is growing in various industries. However, as described above, the technologies that have been developed and used currently have rather limitations due to specialties that have been studied to meet characteristics of the respective technical fields and therefore have trouble in applying to more various industries. An example of the problems to be solved by the 3D imaging technology that may be applied to more various industries may include detecting more various samples by reducing a limitation on a material, a size, or the like. Above all, what is most urgent for the 3D imaging technology is to implement the high-speed and high-precision measurement.
Among the foregoing technologies, the imaging technology using the THz beam may acquire a 3D image by being combined with other technologies. Compared to the X-ray CT technology, the imaging technology using the THz beam does not damage a sample and therefore has higher safety and compared to the OCT technology, the imaging technology using the THz beam may detect a sample having a much larger depth, and so on. That is, the THz beam may have several characteristics that may be appropriately used for a next generation high-speed 3D imaging technology. However, researches for the imaging technology using the THz beam are still in beginning stages, and therefore the imaging technology using the THz beam has many problems to be solved.
A method for obtaining a 3D image using a THz beam is classified into a transmission type and a reflection type. The transmission type is similar to the foregoing CT technique. However, the transmission type uses the THz beam instead of using the X-ray, and therefore may have high biological safety but still has a problem of too long measurement time that is the largest problem with the CT technology. The reflection type uses a time-of-flight (TOF) principle. That is, the reflection type calculates a distance based on the returning time of beam when the beam is irradiated to a sample and reflected from the sample to acquire information in a depth direction (that is, beam propagation direction). The THz beam has properties of reflection at interfaces as well as transmission, and therefore the reflection type may calculate a position where a reflected signal is generated by detecting the reflected signal to understand position information on the interfaces present in a depth direction of a sample and investigate the depth direction information on multiple points on a 2D plane perpendicular to the depth direction, thereby acquiring the 3D shape information in the sample.
FIG. 1 is a diagram schematically illustrating the existing reflection type 3D imaging system using a THz beam, in which the 3D imaging system irradiates the THz beam to a sample to acquire a reflected signal and moves the sample two-dimensionally. In more detail, a detailed content thereof is disclosed in “High-speed terahertz reflection three-dimensional imaging for nondestructive evaluation” (Kyong Hwan Jin, Young-Gil Kim, Seung Hyun Cho, Jong Chul Ye, Dae-Su Yee, 25 Nov. 2012/Vol. 20, No. 23/OPTICS EXPRESS, hereinafter, related art document 1).
As described in the related art document 1, the reflection type 3D imaging system may detect the high-precision 3D image using the THz beam, and as a result achieve considerable technical development in the 3D imaging technical fields. However, since the sample needs to be physically moved on the 2D plane, the reflection type 3D imaging system disclosed in the related art document 1 still has a limitation in speed and precision. As a result, there is a need to increase the speed. In addition, as described in the related art document 1, the reflection type 3D imaging system uses the pulse wave THz beam to measure the reflected signal in the time domain and therefore needs to use femtosecond pulse lasers to generate and measure the pulse wave THz beam. As a result, the reflection type 3D imaging system may have a problem in that it takes much cost to configure the imaging system and it is difficult to miniaturize the system.
As the related art document disclosing a method for generating a terahertz continuous-wave, of which the frequency varies at a high speed, using a wavelength-fixed laser and a wavelength-swept laser, there is Korean Patent No. 10-1453472. However, the method suggests simply a technology generating a terahertz continuous-wave of which the frequency varies at a high speed and does not yet suggest a technology of measuring a 3D image using the generated terahertz continuous-wave.